The liner of pressure-sensitive products can be of different types, based on a film, a paper, or a combination of both. For self-wound tapes, the liner must be silicone coated on both sides.
Film liners can be silicone coated on both sides, and are stable with respect to the ambient air humidity. However, they are expensive and consequently of limited use. Only when the cost of the total construction can justify the price differential between the subsequently described alternatives, can this solution be implemented commercially. Paper liners, generally a glassine (highly calendered) type of paper, can also be easily silicone coated on both sides, are relatively inexpensive, but suffer regarding ambient air humidity characteristics. Composite liners, made out of paper and film, can easily be silicone coated on both sides, but are either not stable to ambient air humidity or are expensive.
Alternative solutions can involve the use of a clay-coated kraft liner, which is widely used for single-side pressure sensitive labels, etc. The main advantages of this type of liner is that it lies flat, possesses good dimensional stability and obtains these properties at a reasonable price. The clay coating provides a smooth surface with good hold out, an important characteristic for applying a thermally or radiation curable silicone coating, by any technique.
In order to use this type of liner for self-wound tapes, it traditionally would have been clay-coated on both sides, to provide enough hold-out thereby allowing silicone coating on both sides without a great deal of saturation of the paper. However, a problem which arises when the same coating is applied to both the front and back sides of this type of liner, is that a "bursting" of the paper is likely to occur during passage of the liner through the drying ovens. The "bursting" is induced by the moisture contained in the paper which cannot come out during the thermal drying/curing of the silicone coating as these clay coatings are relatively impervious to water vapor migration. Moreover, this characteristic does not allow proper remoisturization of the liner after the drying/curing of the silicone, which is an essential step to keep the targeted final product characteristics, particularly good dimensional stability (i.e., the product will lay flat).
Standard one-side clay-coated kraft has also been tested in prior art solutions, but requires a high silicone coating weight, sufficient to completely soak the liner with silicone. This mitigates against the economic attractiveness of the process. Additionally, such a heavy silicone coating weight makes it impossible to properly remoisturize the liner after drying/curing of the silicone.
Thus, as can be seen, prior art solutions to the problem have yet to satisfactorily solve the technical issues facing the manufacture of smooth double silicone-coated liners, which are free from undesirable defects on the surface.